1. Field of the Invention
The present invention relates to apparatus and methodology for ablating tissue and, more particularly, to determination of an unambiguous formation of a lesion having a predictable depth and volume at an ablation site.
2. Background of the Invention
The heart is a four chamber muscular organ (myocardium) that pumps blood through various conduits to and from all parts of the body. In order that the blood be moved in the cardiovascular system in an orderly manner, it is necessary that the heart muscles contract and relax in an orderly sequence and that the valves of the system open and close at proper times during the cycle. Specialized conduction pathways convey electrical impulses swiftly to the entire cardiac muscle. In response to the impulses, the muscle contracts first at the top of the heart and follows thereafter to the bottom of the heart. As contraction begins, oxygen depleted venous blood is squeezed out of the right atrium (one of two small upper chambers) and into the larger right ventricle below. The right ventricle ejects the blood into the pulmonary circulation, which resupplies oxygen and delivers the blood to the left side of the heart. In parallel with the events on the right side, the heart muscle pumps newly oxygenated blood from the left atrium into the left ventricle and from there out to the aorta which distributes the blood to every part of the body. The signals giving rise to these machinations emanates from a cluster of conduction tissue cells collectively known as the sinoatrial (SA) node. The sinoatrial node, located at the top of the atrium, establishes the tempo of the heartbeat. Hence, it is often referred to as the cardiac pacemaker. It sets the tempo simply because it issues impulses more frequently than do other cardiac regions. Although the sinoatrial node can respond to signals from outside the heart, it usually becomes active spontaneously. From the sinoatrial node impulses race to the atrioventricular (AV) node above the ventricles and speeds along the septum to the bottom of the heart and up along its sides. The impulses also migrate from conduction fibers across the overlying muscle from the endocardium to the epicardium to trigger contractions that force blood through the heart and into the arterial circulation. The spread of electricity through a healthy heart gives rise to the familiar electrocardiogram. Defective or diseased cells are electrically abnormal. That is, they may conduct impulses unusually slowly or fire when they would typically be silent. These diseased cells or areas might perturb smooth signaling by forming a reentrant circuit in the muscle. Such a circuit is a pathway of electrical conduction through which impulses can cycle repeatedly without dying out. The resulting impulses can provoke sustained ventricular tachycardia: excessively rapid pumping by the ventricles. Tachycardia dysrhythmnia may impose substantial risk to a patient because a diseased heart cannot usually tolerate rapid rates for extensive periods. Such rapid rates may cause hypotension and heart failure. Where there is an underlying cardiac disease, tachycardia can degenerate into a more serious ventricular dysrhythmia, such as fibrillation. By eliminating a reentrant circuit or signal pathway contributing to tachycardia, the source of errant electrical impulses will be eliminated. Ablation of the site attendant such a pathway will eliminate the source of errant impulses and the resulting arrhythmia. Mapping techniques for locating each of such sites that may be present are well known and are presently used.
Interruption of the errant electrical impulses is generally achieved by ablating the appropriate site. Such ablation has been performed by lasers. The most common technique used at an ablation site involves the use of a probe energized by radio frequency (RF) radiation. Radio frequency (RF) catheter ablation is an effective therapy for the treatment of sustained supraventricular tachycardias such as that due to an accessory pathway. (Jackman, et al. "Catheter ablation of accessory AV pathways (Wolff-Parkinson-White Syndrome) by radiofrequency current", N. Engl J. Med 1991;324:1605-1611; Calkins, et al. "Diagnosis and cure of the Wolff-Parkinson-White syndrome or paroxysmal supraventricular tachycardias during a single electrophysiology test", N. Engl J. Med 1991;324:1612-1618; Kuck et al.;"Radiofrequency current catheter ablation of accessory atrioventricular pathways", Lancet 1991;337:1557-1561; Lesh et al., "Curative percutaneous catheter ablation using radiofrequency energy for accessory pathways in all locations; Results in 100 consecutive patients", J. Am. Coll Cardiol 1992;19:1303-1309; Lee et al., "Catheter modification of the atrioventricular junction with radiofrequency energy for control of atrioventricular nodal reentry tachycardia", Circulation 1991;83:827-835; Jackman et al., "Treatment of supraventricular tachycardia due to atrioventricular nodal reentry by radiofrequency catheter ablation of slow pathway conduction", N.Eng J. Med 1992;327:313-318; Kay et al., "Selective radiofrequency ablation of the slow pathway for the treatment of atrioventricular nodal reentrant tachycardia. Evidence for involvement of perinodal myocardium within the reentrant circuit", Circular. 1992,85(5):1675-88; Jazayeri et al., "Selective transcatheter ablation of the fast and slow pathways using radiofrequency energy in patients with atrioventricular nodal reentry tachycardia", Circulation 1992;85:1318-1328; Klein et al., "Radiofrequency catheter ablation of ventricular tachycardia in patients without structural heart disease", Circulation 1992;85:1666-6174; Nakagawa et al., "Radiofrequency catheter ablation of idiopathic left ventricular tachycardia guided by a Purkinje potential", Circulation 1993;88:2607-2617.) The treatment of atrial fibrillation and ventricular tachycardia by catheter ablation requires longer or deeper lesions. If lesion formation below an electrode could be accurately monitored during its formation, it could improve the ability to produce a continuous line of lesions that is required for ablation of atrial fibrillation. Also, creating deeper lesions could enhance the success of ablation of ventricular tachycardia.
Measurement and control of the applied RF energy is through a thermistor (or it could be a thermocouple) located proximate the RF element at the tip of a catheter probe. While such a thermistor may be sufficiently accurate to reflect the temperature of the thermistor, it is inherently inaccurate and imprecise in determining the temperature of the tissue at the ablation site. (Hindricks, et al., "Radiofrequency coagulation of ventricular myocardium: Improved prediction of lesion size by monitoring catheter tip temperature", Eur Heart Journal 1989;10:972-984; Langberg et al., "Temperature monitoring during radiofrequency catheter ablation of accessory pathways", Circulation 1992;86:1469-1474; Haines et al., "Observation on electrode-tissue interface temperature and effect on electrical impedance during radiofrequency ablation of ventricular myocardium", Circulation 1990;82:1034-1038; Blouin, et al., "Assessment of effects of a radiofrequency energy field and thermistor location in an electrode catheter on the accuracy of temperature measurement", PACE 1991; Part I 14:807-813.) This results from several causes. First, there is a temperature loss across the interface between the ablation site (usually variable due to position of electrode) and the surface of the RF tip. Second, the flow of blood about the non-tissue contact portion of the conductive RF tip draws off heat from the ablation site which causes the thermistor to be cooler than the tissue under ablation. (McRury, et al., "Temperature measurement as a determinant of tissue during radiofrequency catheter ablation: an examination of electrode thermistor positioning for measurement accuracy", J. Cardiovasc Electrophysiol 1995;6(4):268-78; Runbrecht et al., "Influence of flow on intratissue temperature in radiofrequency catheter ablation" (abstract) Circulation 1997;96(8):I-143.) However, temperatures above 100.degree. C. causes coagulum formation on the RF tip, a rapid rise in electrical impedance of the RF tip, and excessive damage to the endocardium. Third, there is a lag in thermal conduction between the RF tip and the thermistor, which lag is a function of materials, distance, and temperature differential. Each of these variables may change constantly during an ablation procedure.
To ensure that the ablation site tissue is subjected to heat sufficient to raise its temperature to perform irreversible tissue damage, the power transmitted to the RF tip must be increased significantly greater than that desired for the ablation in view of the variable losses. Due to the errors of the catheter/thermistor temperature sensing systems, there is a propensity to overheat the ablation site tissue needlessly. (He, et al., "Temperature monitoring during RF energy application without the use of the thermistors or thermocouples", (abstract) PACE 1996;19:626; He et al., "In vivo experiments of radiofrequency (RF) energy application using bio-battery-induced temperature monitoring", (abstract) J. Am Coll Cardiol 1997; 29:32A; Sharma et al., "Bio-battery to monitor temperature during radiofrequency energy application", (manuscript submitted) 1997.) This creates three potentially injurious conditions. First, the RF tip may become coagulated. Second, tissue at the ablation site may "stick to" the RF tip and result in tearing of the tissue upon removal of the probe. This condition is particularly dangerous when the ablation site is on a thin wall of tissue. Third, inadequate tissue temperature control can result in unnecessary injury to the heart including immediate or subsequent perforation.
When radio frequency current is conducted through tissue, as might occur during a procedure of ablating a tissue site on the interior wall (endocardium) of the heart with a radio frequency energized electrode or tip of a catheter, heating occurs preliminarily at the myocardial tissue interface with the tip of the catheter. Given a fixed power level and geometry of the catheter probe, the temperature gradient from the probe interface and a distance, r, into the tissue is proportional to 1/r.sup.4. Heating is caused by the resistive (OHMIC) property of the myocardial tissue and it is directly proportional to the current density. As may be expected, the highest temperature occurs at the ablation site which is at the interface of the RF tip and the tissue.
When the temperature of the tissue at the ablation site approaches 100.degree. C., a deposit is formed on the RF tip that will restrict the electrical conducting surface of the RF tip. The input impedance to the RF tip will increase. Were the power level maintained constant, the interface current density would increase and eventually carbonization would occur. At these relatively extreme temperatures, the RF tip will often stick to the surface of the tissue and may tear the tissue when the RF tip is removed from the ablation site.
To effect ablation, or render the tissue nonviable, the tissue temperature must exceed 50.degree. C. If the parameters of the RF tip of a catheter are held constant, the size and depth of the lesion caused by the ablation is directly proportional to the temperature and time at the interface (assuming a time constant sufficient for thermal equilibrium). In order to produce lesions of greatest depth without overheating the tissues at the interface, critical temperature measurement techniques of the RF tip are required.
The current technology for measuring the temperature of an RF tip embodies a miniature thermistor(s) located in the RF tip of the probe. The present state of the art provides inadequate compensation for the thermal resistance that exists between the thermistor and the outer surface of the RF tip, which may be in variable contact with the tissue and affected by blood cooling or between the outer surface of the RF tip and the surface of the adjacent tissue. Because of these uncertainties contributing to a determination of the specific temperature of the tissue at the interface, apparatus for accurately determining when ablation actually occurs would be of great advantage in performing an electrophysiological procedure to ablate a specific site(s) of the myocardial tissue to a predeterminable area and depth (or volume).